Amazed to hear this! Yep that’s actually possible, you would need only one relay and a handful of diodes to make a simplest one relay automatic battery charger circuit.
How the Circuit Works
The idea struck me while trying to design the easiest possible battery charger circuit for one my clients.
The concept is simple; just raise the operating or triggering voltage of the relay up to the optimal battery charging threshold voltage by dropping the required amount of supply voltage to the relay coil, with the help of series diodes.The idea may be understood from the following points:
Take an ordinary relay, measure its triggering voltage by carefully applying a variable voltage across its coil.Now suppose the triggering voltage of the particular relay was about 9 volts, and also assume you want to raise its voltage to 14 volts, which may be your 12 volt battery’s charging threshold voltage.
We know that a 1N4007 diode drops about 0.6 volts across it, so if we add sufficient number of diodes in series with the relay coil would hopefully pull its tripping or triggering voltage to about 14 volts.
That means, 14 – 9 = 5, we’ll require 5/0.5 = 10 diodes in series to achieve this rise in the triggering voltage of the relay.That’s pretty simple and interesting isn’t it?The rest may be done with the help of the shown diagram…..your simplest single relay automatic battery charger is ready.
Ananda says
Dear Swagatam,
To work this charger correctly it is needed to connect the discharged battery first before give the supply (14 volts) to the charger. Am I correct?
Ananda
Ahmed Elmasry says
i take your idea and made this circuit :http://im75.gulfup.com/3SLo5.png
Swagatam says
Actually, I was wrong in acknowledging the zener idea, it won't work.
It has to be done exactly as suggested in the diagram in the above article. 1N4007 or 1N4148 in series with the relay coil is the only correct way of doing it.
Anonymous says
As soon as the battery charged the relay starts to switch on and off and on and off until you cut off the connection lol.
Farhan Ali says
yes relay starts to switch on and off. i have made this circuit.this is due to disconnection. it order to overcome this connect 14v dc to battery positive terminal not through the NC of relay.
Anonymous says
This idea won't work very well, as the relay will have a very large hysterisis, as well as the diodes being sensitive to temperature.
With the cost of batteries going up dramatically, you really should try hard to avoid over or undercharging them. This circuit will either over or undercharge your expensive battery.
Rajagilse says
Sir. Weather The forward voltage drop of 1n 4007 is always 0.5 volts or it may vary with current flowing through it?
Rajagilse says
Sir, change means, whether the forward voltage of the diode increases with the heat of diode or decreases? Is there any formula of forward voltage drop with reference to current flowing in it.
Debraj says
generally 12V relays hold on to NC even if the voltage to the coil drops to 6V .
wouldn't using a simple zener diode is more reliable ?
Anonymous says
Can the input be done 12V 1Ampere instead of 14V?
Swagatam says
12v input will not charge a 12V battery
rohit choudhary says
Please share a automatic cut off circuit so that , I can charge a mobile battery , with mobile charger.
Swagatam says
you can try the second circuit published in this article:
https://homemade-circuits.com/2012/07/make-6v-4ah-automatic-battery-charger.html
Ahmed Elmasry says
hi i take your idea and make this circuit …
http://im75.gulfup.com/3SLo5.png
Swagatam says
Actually, I was wrong in acknowledging the zener idea, it won't work
It has to be done exactly as suggested in the diagram in the above article. 1N4007 or 1N4148 in series with the relay coil is the only correct way of doing it.
Unknown says
Sir, thank you for sharing your idea on this simple circuit.
What components will be required if I need this same simple circuit to charge 48 volts battery made from 4 x 12. The battery AH is not important.
anticipating your quick response
Thank you once more
Swagatam says
sorry this circuit is not recommended for a 48V battery because too many diodes will be required for the 48v operation …….moreover the design is not too accurate.
untung suharto says
Hi, friend this is can workout well charge SLA battery?
Swagatam says
yes it will work….